Propylene oxide is a colorless and highly reactive material having a molecular formula of CH3CHCH2O. Since the propylene oxide is a volatile, low boiling point, and flammable liquid in terms of chemical properties, the propylene oxide is likely to be exploded by heat or flame and is thus very dangerous. Also, the propylene oxide has the feature of being well mixed with water and most organic solvents, and reacting vigorously with hydrogen chloride, chlorosulfonic acid, and hydrogen fluoride. The propylene oxide is most widely used as a raw material for synthesis of polyether polyol. In addition, the propylene oxide is used in various fields as a raw material for propylene glycol (PG), glycol ethers, butanediol, surfactants, starch and the like. In particular, with growth of markets for polyurethane and propylene glycol using polyol as a raw material, demand for the propylene oxide is also expected to increase steadily by 3-4% per year. Considering factors such as an increase in propylene cost due to an increase in oil price since the 2000s, it is expected that the importance of propylene oxide becomes higher and higher.
Most of propylene oxides so far have been prepared by a chlorohydrin process (see Patent Document 1) using chlorine as a raw material through propylene indirect oxidation, and a hydroperoxide process using an organic peroxide as a raw material (see Patent documents 2 and 3). However, the propylene indirect oxidation is a disadvantageous in that byproducts are necessary to be separated, refined and recycled, in addition to environmental problems or in that ensuring of sales lines is necessary.
Therefore, a novel process for improving the occurrence of byproducts, which is a disadvantage of the commercialization process, and for preparing an environmental-friendly propylene oxide has been developed. Although a HPPO process (see Patent Document 4 and Non-Patent Document 1), in which the propylene oxide was synthesized by oxidizing propylene by using hydrogen peroxide as a raw material, and only water was produced as a byproduct, was developed by Dow, BASF and Degussa, there was a limitation in preparing a large amount of propylene oxide due to a high cost of hydrogen peroxide used as a reactant.
In all of the above-described processes, the propylene oxide is synthesized by using a reactant other than oxygen. This provides an advantage of a high yield, but leads to a disadvantage that the reactant is expensive and additionally produced byproducts should be processed. Therefore, the necessity of a process for preparing a propylene oxide through a direct oxidation reaction between propylene and oxygen without using additional reactants is emerging.
Since only propylene and oxygen are used as raw materials in the process for preparing the propylene oxide through propylene direct oxidation, this process is advantageous in that the raw materials may be easily supplied and received, and the propylene oxide may be synthesized without producing additional byproducts. However, it is difficult to obtain the propylene oxide at a high yield due to a very low conversion rate and selectivity, so that there is no commercialization process. It has been known that this is because the complete oxidation reaction in which water and carbon dioxide are produced during the oxidation reaction of propylene is thermodynamically more stable than the partial oxidation reaction in which propylene oxide is produced. Thus, the complete oxidation reaction occurs in most cases in the absence of catalyst during the reaction between propylene and oxygen, so that it is disadvantageous in preparation of propylene oxide.
In general, as a catalyst for propylene direct oxidation reaction, studies have been conducted to improve the performance of a catalyst used for preparing an ethylene oxide through a reaction between ethylene and vapor-phase oxygen (see Non-Patent Document 2). However, it has been known that commercially feasible results cannot be obtained when this catalyst is applied to the propylene direct oxidation reaction by using catalysts and reaction conditions for ethylene direct oxidation reaction, which is currently commercialized, Therefore, studies on a particular catalyst for propylene direct oxidation have been conducted.
First, a silver catalyst prepared through a precipitation method by introducing various kinds of alkaline (earth) metal halides (NaCl, BaCl2, LiCl, and NH4Cl) without a support exhibits a propylene conversion rate of 14% and a propylene oxide selectivity of 15-35% (see Non-Patent Document 3). However, improved effects are not visibly shown in an aspect of the reaction activity, and it can be said that it is disadvantageous in that silver is used at a very high proportion of 95% by weight in the prepared catalyst. In addition, it is observed that a silver-copper chloride (Ag—CuCl) catalyst (see Non-Patent Document 4), in which copper chloride (CuCl) is introduced as a co-catalyst, had a possibility of preparation for propylene oxide; however, there is a limitation that improved effects is not greatly achieved in an aspect of the reaction activity.
On the other hand, it is observed that a silver catalyst in which a chloride salt and a molybdenum oxide are introduced as a promoter without a support exhibits a propylene conversion rate of 6.8% and a propylene oxide selectivity of 53% (see Non-Patent Document 5). It can be said that this study is excellent in that an experiment was conducted without additional promoting gas and it is reported that molybdenum oxide exhibits an effect as a promoter in addition to chloride salts or alkaline (earth) metals which have been studied in other researches. However, it can be said that the above catalyst still has a disadvantage in that expensive molybdenum and silver were respectively used in a large amount, e.g., 50 wt %, in the prepared catalyst.
Therefore, studies for introducing various supports have been conducted in order to improve the yield of propylene oxide and to minimize the amount of silver, which is an active metal, in the preparation of catalysts.
A silver catalyst prepared by adding a potassium salt promoter to an alkaline earth metal carbonate support exhibits a propylene conversion rate of 34% and a propylene oxide selectivity of 47% in a propylene direct oxidation reaction, resulting in improved reaction activity (see Patent Document 5). However, a relatively expensive silver (40 wt %) is still used in this study, and this catalyst is not favorable economically because additional organic chloride promoting gas is added during the reaction.
A silver catalyst, in which a metal chloride salt and a molybdenum oxide are introduced as a promoter to a zirconium oxide support exhibits a propylene conversion rate of 2.0% and a propylene oxide selectivity of 65.0% (see Non-Patent Document 6). In this study, a relatively low proportion (20 wt %) of silver is used unlike other studies, and it is reported that this catalyst is improved in terms of propylene oxide selectivity. However, a conversion rate is low as compared to a high selectivity, so that it is disadvantageous in terms the yield of propylene oxide.
In addition, there are a silver catalyst prepared by adding an Y2O3 promoter to an α-Al2O3 support (see Non-Patent Document 7), a silver catalyst prepared by adding a MoO3 promoter to a ZrO2 support (see Non-Patent Document 8), etc. All of these catalysts are composed of a low amount of silver compared with the catalysts prepared without the conventional support, but still have a disadvantage in terms of the selectivity and yield of propylene oxide. Thus, it is necessary to study the effect of the support on the propylene direct oxidation.
Meanwhile, it is important to understand a partial oxidation mechanism in the silver catalyst for the effective conversion of propylene oxide through the propylene direct oxidation reaction (see Non-Patent Document 9). In order to produce propylene oxide in this reaction, the oxygen adsorbed at an active site of silver should react with carbon located at a double bond of propylene, which is a reactant, to cause a partial oxidation reaction. However, it is known that since the reactivity of the allylic hydrogen present at a methyl group of propylene is very high, the complete oxidation reaction, in which carbon dioxide and water are generated through the reaction between oxygen adsorbed at silver and allylic hydrogen, dominantly occurs and thus the propylene oxide is not effectively produced. Therefore, when the oxygen adsorbed at the silver is not reacted with allylic hydrogen present at the methyl group of propylene but selectively reacted with carbon located at the double bond of propylene, a high yield of propylene oxide may be expected.